Devices in the semiconductor industry continue to advance toward higher performance, while maintaining or even lowering the cost of manufacturing. Micro-miniaturization of semiconductor devices has resulted in higher performance, through increases in transistor speed and in the number of devices incorporated in a chip; however, this trend has also increased yield and reliability failures. As contact or via openings decrease in size, the aspect ratio, or the ratio of the depth of the opening to the diameter of the opening, also increases. With a higher aspect ratio, the use of aluminum-based metallization to fill the contact opening, results in electromigration and reliability failures. To alleviate reliability failures, the semiconductor industry has evolved to the use of tungsten, in certain devices, for filling narrow but deep contact or via openings.
The switch to tungsten filled contact openings takes advantage of the improved conformal, or step, coverage that results from the use of a plasma enhanced chemical vapor deposition (PECVD) process. In addition, tungsten's high current carrying characteristics reduce the risk of electromigration failure. The conventional method of forming tungsten plugs in vias includes plasma etching of vias or contacts, photoresist striping and cleaning, adhesion layer and barrier metal deposition by physical vapor deposition (PVD) and tungsten deposition by PECVD. Typical adhesion and barrier materials used may consist of a stack of titanium and titanium nitride, respectively. The titanium reduces the contact resistance of the interconnect, and the titanium nitride is a protective layer against titanium attack by a tungsten hexaflouride gas that is used during tungsten deposition. In addition, tungsten adheres to titanium nitride very well, resulting in a mechanically stable tungsten plug.
Unfortunately, after tungsten plug filling, voids, or so-called tungsten seams, are often observed in the tungsten material. This is particularly the case when the etched via profiles are straight. Such tungsten seams are commonly exposed during subsequent processing, such as during processes designed to remove unwanted tungsten from regions other then the contact opening. Moreover, in certain situations the size of the tungsten seam is increased due to exposure to the removal process. This often creates a difficult topology for subsequent metallization coverage as well as electrical device degradation, which is especially apparent as leakage in metal-oxide-metal MOM capacitor structures.
Therefore, processes have been developed, either attempting to create seamless tungsten contact opening fills or repairing the seam or void in the tungsten fill. For example, one attempt involves altering the via etch profile so as to assume a tapered profile, thereby reducing the tungsten seam and allowing better tungsten fill. The tapered via profile helps reduce many of the tungsten seam issues, however, it often leads to increased contact resistance, which is also very undesirable.
Accordingly, what is needed in the art is a contact structure and method of manufacture therefor that does not experience the tungsten “seam” problems, as experienced in the prior art.